Does system.out.println have to be on edt?

Question

I was running some code that did a printout in a swingworker. I was not getting printouts so I used SwingUtilities.invokeLater and now it works. I did not expect this result, how did this happen? I would have thought System.out.println could run outside of the EDT.

Something else is wrong. System.out.println(...) does not care what thread it's called in and only *Swing* method calls need be called on the Swing event thread. — Hovercraft Full Of Eels, Jun 28 '12 at 20:48
nooo System.out.println... that is the _root of all evil_ and also ugly as hell. You can choose between at least 3 easy to setup, lightweight logging frameworks in java. (slf4j, log4j, logback, etc) — Gergely Szilagyi, Jun 28 '12 at 20:52

score 2 · Accepted Answer · answered Jun 28 '12 at 21:03

That would have been pretty easy to test (not to say, even typing all the code to test this is less work then posting it here):

import java.awt.EventQueue;
public class HelloWorld {
  public static void main( String[] args ) {
    System.out.println("Hello world");
    System.out.println( EventQueue.isDispatchThread());
  }
}

results in

Hello world
false

on the console.

So yes, System.out.println can be used outside the EDT

score 1 · Answer 2 · answered Jun 28 '12 at 20:30

1

I would have thoughtsystem.out.println could run outside of the edt.

That is true. To test this, create a thread where you put a loop and a printout and see for yourself :)

answered Jun 28 '12 at 20:30

GETah

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score 0 · Answer 3 · answered Jun 28 '12 at 20:35

System.out.println does run outside the edt. You made it run inside when you ran it using a swingworker. In theory you should always be able to just print your results right from your code block. I suggest:

Runnable runnable = new Runnable() {

        public void run() {

        }
    };
SwingUtilities.invokeLater(runnable);

score 0 · Answer 4 · answered Jun 28 '12 at 21:56

Although it can, it has very few applications outside of debugging.

an example for an alternate output would be JOptionPane:

JOptionPane.showMessageDialog(frame/* sets up the message, can also be replaced with null to remove formatting*/,
"Eggs are not supposed to be green."/* this is your main message*/,
"A plain message"/* this is what shows in the title spot (first parameter must not be null)*/,
JOptionPane.PLAIN_MESSAGE/*shows no icon, also replacable with WARNING_MESSAGE, ERROR_MESSAGE, INFORMATION_MESSAGE*/);

all that would be on one line, but I broke it up for formatting here one line version:

JOptionPane.showMessageDialog(frame/* sets up the message, can also be replaced with null to remove formatting*/, "Eggs are not supposed to be green."/* this is your main message*/, "A plain message"/* this is what shows in the title spot (first parameter must not be null)*/, JOptionPane.PLAIN_MESSAGE/*shows no icon, also replacable with WARNING_MESSAGE, ERROR_MESSAGE, INFORMATION_MESSAGE*/);

without comments:

JOptionPane.showMessageDialog(frame, "Eggs are not supposed to be green.", "A plain message", JOptionPane.PLAIN_MESSAGE);

It's fun to troll your teachers with Dr. Seuss quotes in your programs.

Does system.out.println have to be on edt?

4 Answers4